Computing devices are ubiquitous in modern society. The means by which a human user is to control the computing device is such an important design issue that an entire field of research—that of human-computer interfaces or HCIs—has arisen to provide alternatives. Conventional HCIs use manual input via such devices as keyboards, keypads, mice, touchscreens, joysticks, and the like. However, such manual HCIs are difficult or impossible for people with certain physical disabilities such as motor disorders and paralysis to use effectively. Furthermore, manual HCIs are not well suited for use by users who have their hands occupied with other tasks such as piloting vehicles.
For these reasons, non-manual HCIs have been developed. One emerging category of non-manual HCIs is vision-based controllers. Vision-based controllers typically monitor the eye of the user to track the user's gaze direction and use the resulting coordinate information to control the behavior of the computing device, e.g. for pointing and selection, application switching, password entry, or document navigation. Vision-based HCIs can be designed to replace or be complementary to conventional manual HCIs. Being based on a two-dimensional coordinate location, current vision-based HCIs are well suited to control tasks with a simple two-dimensional movement analogue, such as moving pointers. However, they are not so well-suited to other control tasks that do not have a simple two-dimensional movement analogue, such as controlling the resolution of a visual display of data. Such tasks are typically implemented using dedicated on-screen controls that are manipulable by vision-based control or other means.